The invention relates to a scintillation camera, comprising a scintillation crystal which may comprise a collimator and which serves to convert each photon received into a scintillation, a light guide for coupling the crystal to the entrance window of an array of p photodetectors which serve to convert each scintillation into a current, p acquisition channels which receive the output signals of the photodetectors and which supply p characteristic electric signals which relate notably to the intensity of the scintillations and to the distance between the respective scintillation and each of the photodetectors, and a processor which serves to supply the coordinates x.sub.j and y.sub.j of a scintillation j and its associated energy E.sub.j.
For the determination of the image of the radio-active distribution inside an organ, medical diagnostics utilizes inter alia the scintigraphy principle. This method is based on the introduction of a radioactive element into the organism of a patient which attaches itself more or less to given organs, depending on whether these organs are healthy or not. The measurement of the intensity of the gamma radiation emitted provides an indication of the distribution of the radioactive element in the organism and hence forms a diagnostic aid. A measurement of this kind is performed by means of a scintillation camera.
In conventional scintillation cameras, for example, Anger type cameras (the physician Anger was the first one to propose a scintillation camera whose principles are described in U.S. Pat. No. 3,011,057), the gamma rays which are representative of the radioactive distribution in the enviroment examined penetrate a scintillation crystal after having passed through a collimator. The scintillations thus produced in the crystal are subsequently detected by a series of photomultiplier tubes (for example, 37) after having passed through a light guide which provides optical coupling between the crystal and the tubes. These tubes are distributed in front of the optical block (crystal+light guide) so as to cover substantially the entire surface thereof and to convert the light energy of each scintillation occurring into a measurable electric signal.
Thus, with each photomultiplier tube there is associated an analog acquisition channel which successively provides amplification, integration and shaping of the signals supplied by the tube. The output signals S.sub.ij of the set of acquisition channels are applied to a processor which supplies, by estimation, the coordinates x.sub.j and y.sub.j of a scintillation j and its energy E.sub.j (the index i designates the relevant acquisition channel). The processor may comprise several types of calculation devices, but essentially two thereof are used, in practice, i.e. an arithmetical calculation device for determining the bary center.
In such an arithmetical calculation device, the quantities x.sub.j, y.sub.j, E.sub.j are given by the expressions: ##EQU1## In these expressions: ##EQU2## where the coefficients G.sub.i, K.sub.i, H.sub.i, J.sub.i are weighting factors related to the position of the axis of each of the p photomultiplier tubes.
In such a logarithmic calculation device, the quantities x.sub.j, y.sub.j, E.sub.j are given by the expressions: ##EQU3## The weighting factors are again related to the position of the axis of each of the p photomultiplier tubes.
Regardless of the arithmetic used, contemporary scintillation cameras generally comprise devices for calculating weighted sums which utilize resistance networks with associated summing amplifiers. In the cameras of this type it is not possible to execute calculations relating to a scintillation before the signals corresponding to the preceding scintillation have been set to zero, so that the maximum calculation speed is limited. In order to increase this speed, various solutions have already been proposed, for example, the reduction of the duration of the electric signals or the integration time by means of analog circuits. However, such a reduction could be achieved only at the expense of given intrinsic characteristics of the cameras, notably the spatial and the spectral resolution.
In a previous French Patent Application FR-A 2 552 233 Applicant has proposed a digital radiation measuring device in which it is no longer necessary for the electric signals to return to zero before each new measurement, which means that a partial pile-up of the detected scintillations (and hence of the electric signals or pulses corresponding thereto) is accepted.
It is the object of the invention to propose a novel scintillation camera which incorporates given elements of the above device which, however, are arranged partly within the p acquisition channels and partly within the processor and which has a simplified electronic design which allows for the A/D conversion and the subsequent digital integration of the signals to be performed by means of less accurate, and hence less expensive converters. This design also enables the execution of unpiling calculations by means of a limited number of processing circuits.
To achieve this, the scintillation camera in accordance with the invention is characterized in that:
(A) the p acquisition channels sample the output signals of the photodetectors, followed by the A/D conversion of the samples obtained and their summing, and apply p digital signals to the input of the processor; PA0 (B) the processor itself comprises: PA0 (C) a detection, sequencing and storage stage which receives a signal which corresponds to the sum of the p output signals of the photodetectors is provided in order to supply on the one hand the various clock signals for synchronizing the elements of the p acquisition channels and the elements of the processor, and on the other hand the correction coefficients for the scintillation processing stage.
(a) a bus for transferring the digital signals; PA1 (b) a digital summing stage, comprising four digital weighted sum forming devices which supply four digital signals X.sub.m, Y.sub.m, Z.sub.m, E.sub.m on the basis of the output signals of the p acquisition channels; PA1 (c) a scintillation processing stage which includes unpiling calculation circuits and two dividers and which supplies the three coordinate and energy signals x, y, E on the basis of the signals X.sub.m, Y.sub.m, Z.sub.m, E.sub.m ;
For example, European patent application No. 0166169 describes a scintillation camera which realizes the A/D conversion only in the processor; this notably leads to the use of high-precision and components which are far more costly, i.e. with a ratio of at least 1:100.